Urea-formaldehyde condensation resin solutions are used in large quantities as bonding agents for manufacturing wood materials such as wood chipboards, wood fiberboards, plywood, etc. Wood materials bonded with urea resins have the advantage over those bonded with phenolic resin that they can be manufactured colorless and at a favorable price, but on the other hand they have the big disadvantage of only limited waterproofing and completely insufficient weatherproofing, and in addition continuously set free formaldehyde. Because of these disadvantages, the range of application of wood materials bonded with urea resin is limited.
However, neither do melamine-formaldehyde or acetone-formaldehyde condensation resin solutions produce waterproof and weatherproof wood materials.
Furthermore, wood materials manufactured using phenol-formaldehyde condensation resin solutions are darker and more expensive, but they are water and weather resistant and do not set free formaldehyde. In view of these facts, there has been extended investigation on mixed condensation directed towards strengthening the desirable characteristics of condensation resin solutions as bonding agents for wood materials, and suppressing or at least reducing the undesirable characteristics.
This has also been partly achieved. Thus in DT-PS No. 1 247 017, a urea-acetone condensation resin solution is prepared which gives wood materials improved water resistance. However, the weather resistance is insufficient. Furthermore, this resin sets free formaldehyde because of its urea content.
In DT-PS No. 2 264 288, the weather resistance is improved in that the urea-acetone-formaldehyde resins are after-condensed with phenol-formaldehyde resins, and in DT-OS No. 20 20 481, which relates to a process for preparing a weatherproof wood glue, a bonding agent for weatherproof wood materials is obtained by modifying an aminoplastic resin of melamine, urea and formaldehyde with phenol, but a corresponding urea-free resin possesses substantially better water and weather resistance. The disadvantage of formaldehyde being set free is not eliminated by the known bonding agents for weatherproof wood materials.